The present invention relates to the art of assembling electronic devices, and more particularly, to an improved water washable soldering paste that permits printed circuit boards to be washed with water to remove flux residues.
Soldering pastes have been used for a number of years in the assembly of electronic components, such as integrated circuit packages which are attached to a base member, (i.e. a dielectric substrate) and electrically connected to a conductive circuit formed on or otherwise attached to the base member. A soldering paste is applied to selected regions of the conductive circuit by a screening process. Electronic components are then positioned on the base member and the entire assembly is fired in a heating chamber to bond the components to the conductive circuit.
A soldering paste may contain a soldering flux that functions to eliminate oxide contamination on the solder and on metal surfaces contacting the solder. Many of the organic components in soldering pastes and fluxes are vaporized during the soldering process, however some components leave residues that must be removed by organic solvents, specifically chlorofluorocarbons.
Attempts have been made in the past to develop soldering pastes and fluxes that leave no residue or leave a residue that can be removed with water, however these attempts have been unsuccessful for various reasons.
U.S. Pat. No. 2,898,255, granted Aug. 4, 1959 to Thompson et al., discloses the use of formic acid as a key component in a soldering flux which is reported to exhibit superior oxide removal. Formic acid boils at 100.5.degree. C. and therefore does not remain as a corrosive residue subsequent to the soldering process. However, formic acid cannot be used in a soldering paste because upon prolonged contact with solder, the acid appears to generate lead and tin formates that prevent reflow, namely the ability of the solder to wet the metal surfaces to be bonded.
U.S. Pat. No. 3,223,561, granted Dec. 14, 1965 to Durham et al., discloses the use of an amine salt in a soldering flux. Although use of an amine salt eliminates the damage that occurs when an acid such as formic acid contacts the solder powder, the amine salt deposits a water insoluble white residue which appears to be tin oxide. Such residues are generally recognized as unacceptable.